The simple fact about B&H 16mm 2500 units is that they have plastic worm gears (these are round ridged cylinders that are interlocked with a metallic ridged cylinder) which are normally stressed.  You can tell this by opening the back cover and shining a flashlight on it.  Almost every unit I've purchased had a worm gear that was cracked or just beginning to show signs of wear and tear.  There also exists further into the unit another gear (somewhat smaller).  The larger gear is replaceable, but requires a complete tear down of the unit.  This task will take about 3-4 hours and does require special instruments that are used to properly torque each little pin.  The smaller gear, however, is a different story and if this one goes -  the unit is "kaput".  The nice thing about B&H units is that there's a wealth of them usually on EBAY so if the one you buy does break - throw it away and simply buy another.  Overall, there nice units and very easy to operate which is probably why schools would prefer them over other makes and models.  Now I cannot stress enough that if the seller only states that the unit simply runs forward and reverse abd the lamp comes on - BIG DEAL!  This tells you absolutely nothing about how it projects film.  How do you know the film aperture gates haven't been compromised (which means film will simply slip and chatter and look unstable) or that the sound mechanisms aren't faulty?  So what if it turns on.  I have owned many appliances that power on, but it's after the turn-on that really counts.  if you want me to buy your projector - then the least you can do is invest in a cheap reel of film as a test reel and show me a picture of it running properly.  As to the price you should pay - no more than $100.  After all, you'll be lucky if you get 100 showings out of the typical unit on EBAY so why on earth spend a fortune?